We don’t. Some studies have shown that some people who do not have celiac disease do get sick when eating gluten and get better when not eating it. It’s important to keep in mind, however, that when one removes gluten one also removes large amounts of FODMAP (Fructose, Olygosaccharides, Disaccharides, Monosaccharides And Polyols)-containing foods that are often causing or aggravating symptoms.
See: Biesiekierski JR, et al, Gluten causes gastrointestinal symptoms in subjects without celiac disease: a double blind randomized placebo-controlled trial. Am J Gastroenterol. 2011 Mar; 106(3):508-14; http://www.ncbi.nlm.nih.gov/pubmed/21224837 June, 2013